The present invention relates to an improved ignitor of combustible mixtures in engines. More specifically, the present invention relates to an improved, more durable railplug ignition device that allows significant cooling of the railplug electrodes.
The railplug is a miniaturized railgun that has been developed for use as an ignitor for internal combustion engines. See Harden et al. U.S. Pat. No. 5,076,223, issued Dec. 31, 1991; and Matthews et al. U.S. Pat. No. 5,211,142, issued May 18, 1993. The railplug uses electromagnetic (EM) forces to inject a plasma into a combustion chamber, thereby creating a turbulent plume that acts as a line source for ignition, as opposed to the point source that a conventional spark plug produces. This results in a faster pressure rise during combustion, leading to a better ability than conventional spark plugs to burn mixtures that are difficult to ignite.
FIG. 1 is a cross-sectional illustration of a prior art coaxial railplug as disclosed by Harden et al. The railplug 18 comprises electrodes, or "rails," enclosed in a cavity or chamber similar to the barrel of a gun. In the particular embodiment disclosed in FIG. 1, outer electrode 20 and inner electrode 22 are substantially parallel to each other. Outer electrode 20 is substantially cylindrical in shape and centered around a central longitudinal axis of the inner electrode 22. Outer and inner electrodes 20 and 22, respectively, of railplug 18 are contained within holding plug 24 and surrounded by insulating sleeve 26. Sleeve 26 is substantially cylindrical in shape and electrically isolates outer electrode 20 from holding plug 24. Insulating ring 32 between inner electrode 20 and outer electrode 22 maintains spacing between the electrodes.
Railplug 18, contained within holding plug 24, is designed to open into combustion chamber 30. Like a standard spark plug, holding plug 24 can be threaded into cylinder head 31. Once in place, holding plug 24 provides a connecting interface between railplug 18 and combustion chamber 30. Combustion chamber 30 can be any chamber in which a combustible mixture or medium resides, and for which ignition of that mixture or medium can take place, such as an internal combustion engine or a continuous combustion system. Specific engines and systems include automotive engines with pistons and cylinders, turbine engines, jet engines, etc.
To establish an electromagnetic field, a current path I is shown in FIG. 1 as entering outer electrode 20 and exiting inner electrode 22. A pulsed current source (not shown) is coupled to railplug 18 and provides the necessary current. When a sufficiently high voltage is applied across the rails to electrically break down the gap between them, an arc jumps cross the rails. To complete the current path, arc 34 forms between outer and inner electrodes 20 and 22. During each current pulse, arc 34 begins at plasma initiation point 36 (defined by protrusions 37 and initiation gap 36 at the distal or remote end of railplug 18) and travels towards muzzle end 38 of railplug 18. The action of the current flowing down one rail, across the arc, and back up the other rail produces a magnetic field. The theory is the same as that which describes the magnetic field created in a solenoid. The electromagnetic force or Lorentz force that accelerates the plasma is a consequence of the interaction of the self-induced magnetic field with the current carriers (electrons and positive ions) in the plasma arc.
In FIG. 1, the direction of the magnetic field is perpendicular to the plane of the paper, into the paper below the center electrode 22 and out of the paper above it. The electron flow is up one rail, across the arc, and down the other rail. This current loop results in the Lorentz force, which may be expressed by: EQU F=qv.times.B (1)
where F is the force exerted on a charged particle, q is the charge of the particle, v is the velocity of the particle, and B is the strength of the local magnetic flux density. Alternatively, the electromagnetic force acting on the plasma may be expressed as EQU F=1/2L'I.sup.2 ( 2)
where L' is the inductance gradient or inductance per unit length of the rails and I is current.
FIG. 2B is a cross-sectional view of an alternative prior art embodiment of a railplug. Coaxial railplug 18B comprises outer electrode 20B and inner electrode 22B placed substantially parallel to each other in the bore region from plasma initiation point 35B to muzzle end 38B. Outer electrode 20B is substantially cylindrical in shape and centered around a central axis extending along inner electrode 22B. In the bore region between plasma initiation point 35B and muzzle end 38B, the electrodes are separated from each other by air gap 85. Outer electrode 20B is insulated from holding plug 24B along bore 40B from muzzle end 38B to connection point 86 by air gap 81. At connection point 86, outer electrode 20B is electrically connected to holding plug 24B. It is preferred that the distance S from connection point 86 and plasma initiation point 36B be at least equal to diameter D of bore 40B, and preferably two to three times that diameter. Additionally, it is preferred that the spacing W between outer electrode 20B and inner electrode 22B be approximately equal to the radius of bore 40B, through which the plasma travels from initiation end 36B to muzzle end 38B.
In FIG. 2B, sleeve 26B is substantially cylindrical in shape and electrically isolates inner electrode 22B, which is connected to conductive rod 82, from holding plug 24B. Bushing 84 is threaded into holding plug 24B to retain sleeve 26B. Conductive cap 80 is threaded into sleeve 26B, and is electrically connected to conductive rod 82, and thereby to inner electrode 22B. Other suitable mechanical connections between bushing 84 and holding plug 24B, and also between conductive cap 80 and sleeve 26B, may also be used.
FIG. 2A illustrates a prior art dual-rail railplug 18A, which comprises a pair of substantially parallel electrodes 20A and 22A. Plasma initiation gap 35A is formed by protrusions 37A at the distal end of the chamber formed by bore 40A. Plasma is accelerated from distal or initiation end 36A through gap 35A to muzzle end 38A. Electrode 22A is electrically connected to holding plug 24A at connection point 21A, which is behind plasma initiation point 36A (away from muzzle end 38A).
Several characteristics of railplugs make them promising as engine ignitors. The ability to take full advantage of an electromagnetic force to accelerate the plasma is the most obvious of these. In contrast to other advanced ignitors, thermal expansion augments but does not dominate the jet ejection forces of the railplug. The railplug is designed to maximize the integrated J.times.B product to accelerate the plasma. In addition, the self-induced magnetic field results in an inherently simple design which will enhance reliability and manufacturing economy. Furthermore, since the arc is not stationary as in the case of a plasma jet ignitor, substantial reductions in electrode erosion are contemplated.
Another strong advantage of the railplug over combustion jet or torch ignitors is that it is not necessary to purge the railplug cavity or chamber or to supply fresh mixture--the plasma is created from whatever molecules are present in the railplug cavity at the time of ignition. A further advantage of the railplug is that it generates a relatively large mass of plasma since the arc sweeps down the rails, ionizing essentially all of the gas in its path.
The railplug has been fabricated with dual electrodes in the form of both coaxial and parallel electrodes. FIGS. 2 and 2A show cross-sections of each of these two geometries, respectively, as described in U.S. Pat. No. 5,211,142. It has been found that the coaxial plug is easier to fabricate, as it does not require a complex insulation scheme and because all of its parts are axisymmetric. Commercially available ceramic tubes may be used to insulate the coaxial plug's inner electrode from its outer one. Two nested, staggered tubes may be used to reduce the risk of tracking failure between electrodes. The plug's axisymmetry means that the rear half of the plug may be made identical to a conventional spark plug, and may thus be easily terminated to fit a standard spark plug connector.
The parallel electrode railplug, on the other hand, requires a ceramic sleeve or sidewall 32A as an insulator that is more complex in shape and cannot be easily modified to avoid tracking failures. Further, its lack of symmetry does not make it as easily adaptable to the standard spark plug envelope. However, as discussed below, the structure of the parallel electrode railplug renders it less susceptible to thermal erosion.
For some applications, it may be preferred that one of the electrodes be grounded. In such applications, as illustrated in FIGS. 2, 2A and 2B, the grounding point (21, 21A, 86) is preferably located behind plasma initiation point (36, 36A, 36B) to help avoid balancing the electromagnetic forces acting on the plasma and thus canceling the Loreritz force. In both geometries shown in FIGS. 2 and 2A, one of the rails is grounded to the plug's outer shell 24, 24A, 24B--similar to the manner in which a conventional spark plug is grounded--as opposed to bringing two electrical connections out of the back of the plug. This helps prevent shorting between the electrodes at the high voltages required to break down the gap at elevated pressures (e.g., greater than 18 kV). Also, these designs use an initiation point (36, 36A, 36B) to help ensure that the break down takes place at the same point each time. This may be a welded point, a reduced diameter, a ground flat, or any feature that reduces the gap clearance or increases the electric field stresses such that electrical breakdown occurs preferentially at that point.
The rails or electrodes of the railplug are typically nickel or tungsten with a diameter ranging from 0.040 to 0.100 inch. Tungsten is a particularly suitable material because tungsten welding electrodes are readily available and have a very high melting point, but other materials with high heat transfer characteristics may be suitable as well. The outer electrode or rail in the coaxial railplug configuration is typically brass or steel. Rail lengths in both designs are on the order of 0.5 inch, although other lengths will be apparent to those of skill in the art.
Although the coaxial railplug may be easier to fabricate and adapt for use in conventional spark plug environments, its natural compensation means that it suffers from a relatively low inductance gradient (L'). Typical L' values for the coaxial plug range from about 0.18 to 0.25 .mu.H/m while a typical number for the parallel rail plug configuration is around 0.30 .mu.H/m. The difference could be even greater were it not for the fact that the parallel rails are shielded by the outer shell, which is typically designed to fit a standard 14 mm thread.
Further, the coaxial plug's cantilevered center electrode has relatively poor heat transfer characteristics as compared to the parallel design, where both rails are in intimate contact with surrounding material. This may be an important design parameter from a durability standpoint, as erosion rates increase at higher electrode temperatures.
The foregoing is provided by way of background only. The general state of the art of railplugs is described more fully in the following publications, which are incorporated herein by reference: Harden et al. U.S. Pat. No. 5,076,223, issued Dec. 31, 1991; Matthews et al. U.S. Pat. No. 5,211,142, issued May 18, 1993; R. Faidley, M. Darden and W. Weldon, "The Railplug: A New Ignitor for Internal Combustion Engines," presented at the Sixth EML Symposium, Austin, Tex. (April 1992); R. Matthews et al., "Further Analysis of Railplugs as a New Type of Engine Ignitor," SAE International Fuels and Lubrication Meeting and Exposition, No. 922167 (Oct. 19-22, 1992).
Railplugs have several characteristics that are attractive from the standpoint of combustion ignitors: (1) the energy is deposited over a large surface area; (2) the arc is accelerated down the rails leaving little time available for erosion; (3) the arc is initiated with currents equivalent to those of spark plugs (i.e., the plasma initiation gap 35, 35A, 35B is about the same as a spark gap, or smaller) and then the current is ramped up to higher levels as the arc accelerates down the rails; and (4) suitable materials can be used near the distal or breech end (i.e., the plasma initiation end) of the railplug where wear rates are highest.
Of the railplug configurations described in U.S. Pat. Nos. 5,076,223 and 5,211,142, the coaxial geometry (FIGS. 1, 2, 2B) seems to be the most compatible with present spark plug manufacturing techniques. It also appears to be the least sensitive to contamination of the insulator. However, it has now been discovered that heating of the unsupported center electrode can present a problem. Heat deposited in the center electrode by joule heating and arc heating can be removed only by convection to the surrounding gases and by conduction down the length of the electrode. Unfortunately, enlarging the center electrode diameter to improve the thermal conduction path also decreases the induction gradient of the railplug, which in turn reduces the kinetic velocity achieved for a given current.
Thus, one of the major issues recently discovered in the development of railplugs for commercial applications is the durability of plug electrodes. Presently, plug life in the coaxial design is limited to roughly 10.sup.6 shots. Failure of a plug may be defined as a shorting out of the plug, whether this is due to a tracking failure or a distortion in the shape or position of the center electrode such that it touched the outer electrode (in a coaxial configuration). Plug failure may also be defined as a failure to consistently ignite a mixture. In general, failed plugs exhibit large amounts of erosion around the initiation point, and if the center electrode actually breaks it typically will be in this region.
This thermal management problem in the coaxial design appears to be compounded by the fact that the heating is not uniform along the length of the center electrode, but is concentrated near the arc initiation point where the arc dwells for the longest time. This localized heating results in accelerated erosion of center electrode material at this point leading to premature failure of the railplug.
Specifically, failure of the coaxial railplug is generally known to occur due to material removal from the center rail beginning at the plasma initiation point and extending to a location about 3-4 mm downstream, with almost no erosion further downstream and no significant wear on the outer rail. It is believed that the center rail is becoming too hot in this high erosion region.
Thus, it is desired to have a railplug geometry that incorporates the advantages of the railplug design, but overcomes the thermal management problems described above.